The interaction between the parasite Trichostrongylus tenuis and food quality was investigated in red grouse Lagopus lagopus scoticus, to determine the cause of variation in population cycles between different areas (Scotland and England). Analysis of long-term data, field experiments and population modelling were carried out. Food quality, in terms of nutrient content of heather, was lower on Scottish grouse moors than on English moors. Parasite burdens were also lower in Scottish grouse populations, than in English grouse populations. A three-way interaction, between food, parasites and area (ScotlandlEngland), acting on breeding production, could explain the variation in population cycle period between areas. However, body condition of grouse was not affected by a food-parasite interaction. Experimental manipulation of food quality and parasite burden did not influence the breeding production of female grouse. Modelling the effects of a food-parasite interaction on grouse populations provided evidence that such an interaction could explain variation in cycle period between areas, although other factors are likely to be important in some cases. Red grouse are not unique, as other species also have cycles driven by food and parasites. Other species do not show cyclic population fluctuations because of having shared parasites, and a strong immune response. There is a specialist predator-prey relationship between red grouse and T. tenuis.